1. Field of the Invention
The invention relates to the melting of materials, which are capable of being drawn, such as basaltic glasses, blast furnace slags and the like, with a view of the industrial production of rock wool.
2. Discussion of Background
It is known to convey the material to be formed into fibers, in the molten state, at the periphery of three or four centrifuging wheel, arranged close to each other, the material being poured onto a first wheel which accelerates it and being sent on to the next wheel. Each wheel converts a proportion of the melt and sends the excess on to the next wheel.
These techniques for the production of rock wool from high melting point materials are more generally known as free centrifuging. They have already formed the subject of numerous improvements which are aimed particularly at an improvement in the yield of fibers. It has thus been shown that the percentage of products not converted into fibers is reduced by an optimization of the wheel diameter and speed of rotation of the wheels. Similarly, the surface quality of the wheels affects the drawing of the fibers, according to whether it does or does not promote melt adhesion. Other proposals refer to the circulation of gas around the centrifuging wheels.
Furthermore, French patent application FR No. 84/16,547 proposes to control the conditions under which the melt is conveyed from the melting enclosure to the first centrifuging wheel. For this purpose, a reserve is inserted in the path of the melt, and this ensures homogenization of the temperature and of the composition of this material. In addition, the establishing of this reserve ensures a more uniform flow of the melt originating from the melting enclosure.
In point of fact, the material is commonly melted in furnaces of the "cupola" type, employed in foundry practice. The upper part of the cupolas are charged with successive alternating layers of fuel, generally coke, and of the material to be made into fibers. The combustion zone is situated in the lower region of the cupola, in the vicinity of the tuyeres through which the combustive gas is blown in, generally air, oxygen-enriched if desired. The temperature produced by the combustion causes melting of the material to be made into fibers, and this escapes through a tap hole or conduit situated at a level which is slightly lower than the level of the tuyeres. Lastly, the molten iron residues, originating either from the nodules included in the slag or from the reduction of iron oxides in the remainder of the charge, accumulate at the bottom of the cupola. Although the denser molten iron separates naturally from the melt to be made into fibers, the tap hole must be at a level which is as high as possible in order not to permit a concurrent escape of small quantities of molten iron, which would cause rapid wear of the centrifuging wheels.
Melting enclosures of this kind are highly advantageous, considering the very high flows of the molten products and the low energy cost of this melting. However, these cupolas produce a highly nonuniform flow of molten material, as is emphasized in French patent application Fr No. 84/16,547.
The flow irregularities are due to many causes. In a stationary regime, the level of the combustive and fuel charges is, in fact, substantially constant; however, collapses of charges, especially of coke, take place at uniform intervals, and these result in entrainment of unburned pieces of coke with the melt stream. These coke particles are particularly damaging to the plant, because they increase the quantity of unmelted products and, above all, the fibers produced simultaneously with this coke entrainment have to be removed because of the risk of forming a flammable product.
Furthermore, as indicated earlier, in order to reduce the entrainment of molten iron, the tap hole for the materials to be made into fibers needs to be situated at a relatively high level, in other words in the vicinity of the combustion zone and of the injection tuyeres for the combustive gas. The tap hole for the material to be made into fibers is therefore situated in a region where high turbulence prevails and large quantities of gas or even of solid materials escape in the direction of the centrifuging wheels.